A number of prosthetic devices have been developed to assist upper-body amputees, i.e., persons who have lost all or a portion of an upper limb. Although there is a significant need for such devices, many prosthetic devices (artificial arms and/or hands) suffer from poor performance and design.
One type of prosthetic device commonly used is a “body-powered” system. In a body powered system the wearer controls the system using muscles in his or her body, most commonly muscles of the shoulder and neck. Within the realm of body-powered prostheses, there are two primary families of prehensors, differing primarily in their principle of operation. A prehensor, also known as a “gripper” or an “end-effector,” is a mechanical prosthetic terminal device used by an upper-body amputee to serve as a replacement for the hand. Voluntary opening (VO) prehensors typically include two or more gripping digits (mechanical fingers) that are held or biased against each other by a spring or one or more custom elastic bands. The user moves the digits apart prior to gripping an object by pulling on a control cable connected to the user's shoulder and neck through a harness. When the user relaxes or eases the tension on the control cable, the digits close on the object to be held and grasp the object. In essence, voluntary opening prehensors are spring loaded clamps that can be opened at will by the user. Therefore, with a voluntary opening prehensor, the wearer's grip on the object is passive and the wearer is not required to expend energy while gripping an object. Unfortunately, since gripping an object with a voluntary opening prehensor is passive, the user has limited, if any, control over the amount of force exerted on the object. Gripping forces needed to lift heavy objects are excessive for small, lightweight or fragile objects. Conversely, the correct gripping force needed to grasp a light object will usually be inadequate for heavier objects. U.S. Pat. No. 3,604,017 issued to Brown et al. and U.S. Pat. No. 5,116,386 issued to Scribner disclose voluntary opening prehensors.
Split hook terminal devices (TDs) are one type of VO prehensor because the grasping digits move apart, or open, as the user increases tension in the control cable. Custom elastic bands or springs affixed to the terminal device cause the device to close and effect grasp as cable tension is reduced, identical in principle to a spring-loaded clamp. Many users who desire increased gripping forces install large numbers of elastic bands to generate high pinch force. While pinch force is increased, substantially larger cable tension is required to cycle or open the terminal device to affect grasp. Repetitive actuation of TDs equipped with excessive bands has been strongly implicated in repetitive stress injuries among upper-extremity amputees in addition to causing excessive equipment wear. Similarly, users may offset a portion of the maximum pinch force by sustaining a counter level of cable tension. While reduced pinch force is achieved, prolonged exposure to the sustained cable tension aggravates repetitive stress injuries. Moreover, the elastic bands typically installed in commercially available TDs are specialized, short, powerful bands that are known in the orthotics and prosthetics industry as being notoriously difficult to install, particularly for amputees. Current commercial VO TDs do not lend themselves to readily changing the number of active bands and, therefore, there is no simple or effective way to easily adjust pinch force as needed or desired.
One example of a VO TD with an adjustable tensioning mechanism is German Patent DE901583. A device is shown in which a metal coil spring extends between two digits. One end of the coil spring is attached to a movable digit and the opposite end of the spring is attached to an adjustment mechanism for altering the location where the spring is attached to the fixed digit. The adjustment mechanism permits the spring to be positioned at a number of different locations on the fixed digit that are all equally spaced from the point at which the spring is connected to the movable digit. The tension in the spring does not change between positions. The adjustment mechanism includes a pin (k) which fits in a pin slot (n) to secure the adjustment mechanism to the fixed digit. Each position provides a different pin slot for locating the pin and securing the position of the adjustment mechanism. The closer the pin location is to the pivot point of the movable digit, the less pinch force is applied by the movable digit. This system has numerous drawbacks including the fact that in order to adjust the position of the coil spring, the pin (k) must be grasped and pulled from the pin hole (n) in a direction directly opposite the force applied by the spring. The adjustment mechanism must then be moved to a new position and the pin inserted into the new pin slot. This requires relatively high strength and dexterity. Further, the coil spring extends across the area in which an object may be grasped, possibly interfering with grasping of an object or damaging the object as a result of the metal spring rubbing against the grasped object.